X-rays probe failures in ferroelectric memories

Portland, Ore. - Unlike a conventional random-access memory, a ferroelectric RAM can switch its magnetic domains electrically between zero and one without having to call on standby power. Since ferroelectric memories already store data on smart cards and transform electrical pulses into sound in watch buzzers and ultrasound machines, it might seem that their properties were well-understood. Instead, as an advanced X-ray technique showed recently, the opposite is true.

Conventional wisdom maintained that a single mechanism caused ferroelectric switches to get stuck after a few hundred thousand transitions between one and zero-making them unsuitable ro replace other RAM. But researchers at the University of Wisconsin (Madison), seeking ways to extend ferroelectrics' lifetimes, unveiled at least two mechanisms that cause ferroelectrics to fail.

"People thought that there was a single mechanism that caused ferroelectrics to fail, because they are electrically identical," said Paul Evans, an assistant professor of materials science and engineering at the university, who also works at the Center for Nanoscale Materials, Argonne National Laboratory (Argonne, Ill.). "But using our synchrotron X-ray microdiffraction technique, we were able to visualize directly the evolution of these polarization domains and found two separate mechanisms."

"We can image right through metal or oxide electrodes with submicron resolution, and we found two regimes of fatigue," Evans said. "One was a low-field regime that could be reversed with a higher electric field pulse, and the other was a nonreversible crystallographic relaxation of the epitaxial ferroelectric film when very high electric fields are used."

Ferroelectrics are known to rearrange their atoms to switch from one magnetic pole to the other, but until now researchers had not used a high-energy X-ray to image exactly what makes them work, much less what makes them fail.

At first, almost all the atoms in the magnetic domain changed state. But as the device cycled, larger and larger portions of the domain began to get stuck. Eventually, after as few as 50,000 switches, at a low voltage, nearly all the atoms would get stuck in one state. But they could be put back in motion with a higher-voltage switching pulse.